Conductors having insulation of polyester imide resin

ABSTRACT

Polyester imide resins containing 5-membered imide rings between the ester groups. The resins are soluble in cresol, and cresol solutions are useful as enameling lacquers for wire conductors.

This application is a division of Ser. No. 677,207, filed Apr. 15, 1976, now U.S. Pat. No. 4,170,684. Said Ser. No. 677,207 is a continuation of Ser. No. 293,878 filed Oct. 2, 1972, now abandoned, which is a division of Ser. No. 659,234, filed Aug. 8, 1967, now U.S. Pat. No. 3,697,471 which is a continuation of Ser. No. 238,315, filed Nov. 2, 1962, now abandoned.

The present invention relates to polyester resins, and more particularly to polyester resins which in addition to the ester groups also contain imide groups, and also to a new series of imide group-containing agents which can be used to modify polyester resins.

Synthetic resins produced from polyvalent alcohols and polycarboxylic acids have been known for a long time. These are generally known under the designation "alkyd resins" and provide one of the most important classes of synthetic resins. Alkyd resins have also been known to be modified in various ways, primarily by the incorporation of natural or synthetic fatty acids or their glycerides therein. The particularly temperature stable alkyd resins are however the oil-free alkyd resins of aromatic carboxylic acids, especially of isophthalic acid and terephthalic acid.

Alkyd resins have also been modified by the incorporation therein of nitrogen-containing compounds, for example by the use therein of diamines, amino alcohols or amino carboxylic acids which are reacted with the reactants used for forming the alkyd resins, or by the subsequent reaction of the alkyd resins with polyisocyanates. Products obtained in this manner exhibit improved elasticity and good temperature stability. However, these resins also do not meet all of the necessary requirements for all applications.

For some time linear polyimide resins have been known, these having been obtained by the condensation of diamines with tetracarboxylic acid anhydrides. These resins are marked by an extremely high softening temperature and duration of stability to high temperatures, as well as by a high stability against all normal solvents. This latter advantage actually makes it difficult to make use of and work up these linear polyimide resins. Attempts have been made to avoid this disadvantage by first producing the linear polyamidodicarboxylic acid which is only soluble in relatively expensive solvents and then the polyamidodicarboxylic acid after the fashioning thereof, for example as a lacquer coating, fibers or the like, is converted by heating or splitting off of water by means of suitable chemicals, into a polyimide. However, polyamidodicarboxylic acids can only be made into lacquers in the form of solutions with a very low solid body content, because concentrated solutions thereof possess too high a viscosity for working up. A further disadvantage of the polyamidodicarboxylic acids resides in the fact that the same cannot be contacted with iron, and consequently considerable apparatus expense in the use of expensive noble metals is necessary in order to work with the substances.

It is accordingly a primary object of the present invention to provide a new series of resins which possess advantages not possessed by the above mentioned resins, and which consequently can be used for various important technical purposes.

It is another object of the present invention to provide new polyester imide resins of superior characteristics.

It is yet another object of the present invention to provide for the modification of polyester resins or alkyd resins of all types, and particularly of alkyd resins produced from aromatic polycarboxylic acids, so as to improve the properties thereof.

It is another object of the present invention to provide a new series of modifying agents for the modification of polyester resins, these modifying agents being usable either during or after the production of the polyester resins.

It is still another object of the present invention to provide methods of producing the new polyester imide resins of the present invention.

It is still another object of the present invention to provide lacquers of polyester imide resins for use in various applications, and particularly for use as lacquers for the coating of magnetic wire and the like electrical conductors.

It is still a further object of the present invention to provide insulated electrical conductors which are insulated by means of the polyester imide resins of the present invention.

Other objects and advantages of the present invention will be apparent from a further reading of the specification and of the appended claims.

With the above and other objects in view, the present invention mainly comprises the modification of a polyester resin so that the same contains at least one 5-member imide ring between functional groups of the molecule. Thus, in accordance with the present invention there is provided ester imide resins, particularly polyester resins of one or more polycarboxylic acid and one or more polyvalent alcohol, as well as hydroxy carboxylic acids, and also as well as the use of the functionally active derivatives of the polycarboxylic acids and of the polyvalent alcohols, in which one or more of the starting materials contains one or more 5-membered imide ring between the functional groups of the molecule.

It has surprisingly been found that ester imide resins, which contain not only ester groups but also imide groups, on the one hand contain many properties which are better than those of the known alkyd resins, and on the other hand avoid the difficulties of the polyimide resins. This is accomplished by producing the ester imide resins of polycarboxylic acids, polyvalent alcohols, including hydroxy carboxylic acids or their derivatives, wherein one or more of the starting substances is partly or entirely of such compound that there is one or more 5-membered imide ring between the functional groups of the molecule.

Thus, for example, enameled lacquer films of the ester imide resins of the present invention possess marked thermal stability, increased solvent resistance, hardness and flexibility. Particularly notable are the improvements in the action of the combined mechanical and thermal stresses, such as in the thermal ageing of stretched or compressed lacquer films. Because of these advantageous properties, the products of the present invention are particularly suitable for the production of enameling, particularly for magnetic wires for electrical insulation purposes, since for these purposes materials possessing all of the superior properties of the products of the present invention were not known. Thus, lacquer films of good resistance in the so-called heat shock test were unknown. Also of importance for the use of the resins of the present invention as lacquers for magnetic wires or for electrical insulation purposes is the solubility thereof in cheap solvents such as cresols, or if necessary in mixtures of cutting agents or diluents, such as solvent naphtha. In accordance with the present invention it is possible to produce low viscosity lacquers of high solid body content.

It should also be noted that the ester imide resins of the present invention in the use thereof for enameling lacquers can be improved by the addition of esters of titanic acid or of zirconic acid.

The combination of the advantageous properties of the polyimide resins and of the polyester resins in the new polyester imide resins of the present invention could not be predicted, particularly with respect to the suitable use thereof in the field of electrical insulation of conductors such as copper wires. It is thus quite surprising that even with a slight content of imide groups the properties of the ester imide resins of the present invention is markedly influenced. The use of the ester imide resins of the present invention is, however, not limited to application for electrical insulation purposes. These resins can also be used for all other purposes for which the known alkyd resins could be used, particularly for all typs of coatings, for the production of foils and films, and also for production of molded bodies.

As indicated above, the production of the ester imide resins according to the present invention is carried out by esterifying a polycarboxylic acid (or its functional derivatives) with a polyhydric alcohol as well as with a hydroxy carboxylic acid, if desired, in known manner, whereby in accordance with the present invention one or more of these starting substances contains between the functional groups in the molecule one or more 5-membered imide group. As is clear from the above instead of using the acids or alcohols themselves it is possible to use instead their functional derivatives, such as their esters or semi-esters.

The imide group-containing starting materials of the desired structure which are used for the production of the ester imide resins of the present invention can, for example, be produced by reaction between compounds of the following two classes:

(a) An aromatic carboxylic acid anhydride which besides the 5-membered cyclic carboxylic acid anhydride group also contains at least one additional reactable group. These additional reactable groups can be carboxyl groups, an additional carboxylic acid anhydride group or a hydroxyl group.

In this class of compounds there can be used also, instead of the cyclic carboxylic acid anhydride groups, compounds in which two adjacent carbon atoms contain carboxyl groups, or to use their esters or semi-or half esters, as well as their semi-or half imides with a primary amine of the type set forth under (b) below. Any of these compounds of this type can be used for the imide formation.

(b) A primary amine, which besides the primary amino group also contains at least one additional reactable group. These additional reactable groups can be carboxyl groups, a hydroxyl group, or an additional primary amino group.

Instead of the primary amines it is possible to use their salts, amides, lactams or polyamides, so long as the bound primary amino group is reactable for imide formation.

Examples of compounds of a cyclic carboxylic acid anhydride grouping and an additional functional group falling under (a) above, are primarily pyromellitic acid anhydride and trimelltic acid anhydride. In addition other carboxylic acid anhydrides are suitable, such as naphthalene tetracarboxylic acid dianhydride or dianhydrides of tetracarboxylic acid with two benzene nucleii in the molecule in which the carboxyl groups are in 3,3',4,4'-position.

Examples of compounds with a primary amino group and an additional functional group falling under (b) above, are primarily the aliphatic diprimary diamines such as ethylenediamine, tetramethylenediamine, hexamethylenediamine, nonamethylenediamine, as well as aromatic diprimary diamines such as benzidine, diaminodiphenylmethane, diaminodiphenylketone, diaminodiphenylsulfone, diaminodiphenylsulfoxide, diaminodiphenylether and diaminodiphenylthioether, phenylenediamine, toluylenediamine, xylylenediamine, as well as diamines with three benzene nucleii in the molecule, such as bis-(4-aminophenyl)-αα'-p-xylol or bis-(4-aminophenoxy)-1,4-benzene, and also cycloaliphatic diamines, such as 4,4'-dicyclohexylmethanediamine. Also suitable are amino alcohols such as monoethanolamine, monopropanolamine or dimethylethanolamine, as well as amino carboxylic acids such as glycocol, aminopropionic acids, amino capronic acids or amino benzoic acids.

The reaction products of (a) and (b) above can be reacted with polyester reactants, e.g. a polycarboxylic acid such as terephthalic acid and a polyhydric alcohol or alcohols such as ethylene glycol and/or glycerin to form the ester imide resins of the present invention, or the compounds of (a) and (b) above can be reacted together with the polyester-forming reactants, or a polyester may be reacted with the reaction product of (a) and (b) above or with the compounds of (a) and (b) above themselves. All of these methods of proceeding can be used to produce the ester imide resins of the present invention, as well be further detailed below.

It should be noted that throughout the specification and claims of this case whenever mention is made of a reactable group such as a carboxyl group or the like it is possible to use not only the free functional group itself, but to use its functionally active equivalent, e.g. the ester, instead. Thus, for example, mention may be made of the reaction of terephthalic acid with a polyhydric alcohol to form the polyester. It is apparent that instead of the terephthalic acid itself it is possible, and sometimes even desirable to use its functional equivalents such as a lower dialkyl ester thereof, e.g. dimethyl terephthalate, diethyl terephthalate or the like, or also to use a semi-or half ester such as methyl terephthalate. The use of such functional equivalents which are in general well known in this art is meant to be comprehended within the scope of the present invention, and in this connection too, it should be noted that this discussion applies not only to the functional derivatives of the carboxylic acids, but also of the amines, etc.

Among the most suitable imide group-containing starting materials for the production of ester imide resins according to the present invention are those obtained by the reaction of 2 mols of trimellitic acid anhydride (or its reactable derivatives or functional equivalents) with 1 mol of a diprimary amine (or its reactable derivatives or functional equivalents). The result of this reaction is a diimidodicarboxylic acid which may be used to form the ester imide resins of the present invention.

Likewise suitable is the monoimidedicarboxylic acid obtained by the reaction of 1 mol of trimellitic acid anhydride with 1 mol of an amino carboxylic acid.

A suitable diimidodicarboxylic acid for the production of ester imide resins of the present invention can be obtained by the reaction of 1 mol of a tetracarboxylic acid dianhydride, such as pyromellitic acid dianhydride, with 2 mols of an amino carboxylic acid with primary amino groups or their functional equivalents.

A particularly suitable diimide alcohol which can be used as an imide group-containing starting material for the production of the ester imide resins of the present invention may be obtained by reacting 1 mol of a tetracarboxylic acid anhydride such as pyromellitic acid dianhydride with 2 mols of an amino alcohol with primary amino groups.

A suitable monoimide hydroxy carboxylic acid which may be used according to the present invention may be obtained by the reaction of 1 mol of trimellitic acid anhydride with 1 mol of an amino alcohol with primary amino groups.

In general, for the formation of cyclic imide compounds it is desirable that the used amount of the acid anhydride component (a) and of the amine component (b) are at least approximately equivalent.

As is clear from the above the imide groupcontaining starting material can be obtained by the reaction of an acid component (a) such as trimellitic acid anhydride and an amine component (b) such as a diprimary diamine or an amino carboxylic acid with primary amino groups, or instead of the trimellitic acid anhydride it is possible to use as the acid component (a) a tetracarboxylic acid dianhydride such as pyromellitic acid dianhydride. Instead of the diprimary diamine or the amino carboxylic acid it is equally possible to use an amino alcohol with primary amino groups.

It should also be noted that instead of using a single acid component or a single amine component for the formation of the imide group-containing starting material it is possible, and sometimes even desirable, to use a mixture of such components. Thus, for example, it is advantageous to use instead of trimellitic acid anhydride itself, a mixture of trimellitic acid anhydride with up to 40% thereof substituted by an equivalent amount of pyromellitic acid anhydride. Likewise, it is possible to substitute, for example, up to 30% of a monoalkanolamine with an equivalent amount of a diprimary amine which can either be an aliphatic, aromatic or aliphatic-aromatic diamine.

A particularly advantageous ester imide resin may be produced by the reaction of 1 mol of trimellitic acid anhydride with 0.35-0.8 mols of monoethanolamine and 0.2-2 mols of ethylene glycol. As indicated above, the trimellitic acid may be substituted in part by pyromellitic acid and the monoethanolamine may be substituted in part by an equivalent amount of a diprimary diamine. In addition, the ethylene glycol may be substituted in part, for example up to about 0.4 mols thereof, calculated with respect to 1 mol of trimellitic acid anhydride, with an equivalent amount of a trihydric alcohol, or an alcohol with 4 or more hydroxyl groups.

The following examples A-M relate to the production of imide group-containing starting materials for the production of ester imide resins according to the present invention. The scope of the invention is, of course, not meant to be limited to these specific examples.

EXAMPLE A

115 g (0.6 mols) of trimellitic acid anhydride are introduced into 500 g of a technical cresol mixture at a temperature of 150° C. When all of the trimellitic acid anhydride has been dissolved, 60 g (0.3 mol) of 4,4'-diaminodiphenyl methane are added thereto. This mixture is stirred for 6 hours at 140° C.

Upon cooling a yellowish, finely crystalline precipitate forms which is filtered off and washed several times with alcohol and ether.

The obtained product does not melt at temperatures up to 360° C.

EXAMPLE B

24.8 g (0.1 mol) of 4,4'-diaminodiphenyl sulfone and 38.4 g (0.2 mol) of trimellitic acid anhydride are stirred into 500 g of technical cresol mixture to form a slurry, and this slurry is heated for 4 hours at 175° C. After cooling there is obtained a white, crystalline precipitate which is washed as described in Example A.

The product does not melt at temperatures up to 360° C.

EXAMPLE C

32.4 g (0.3 mol) of p-phenylenediamine and 115.2 g (0.6 mol) of trimellitic acid anhydride are heated with 1200 g of technical cresol mixture for 4 hours at 180° C. After cooling the mixture is filtered and the green crystals constituting the obtained reaction product are washed as described in Example A.

The product does not melt at temperatures up to 360° C.

EXAMPLE D

23.2 g (0.2 mol) of hexamethylenediamine and 76.8 g (0.4 mol) of trimellitic acid anhydride are heated for 3 hours at 190° C. in 400 g of a technical cresol mixture. After cooling and filtering the obtained white crystals are washed with alcohol and ether.

The melting point of the product is 301° C.

EXAMPLE E

20 g (0.1 mol) of 4,4'-diaminodiphenyl ether and 38.4 g (0.2 mol) of trimellitic acid anhydride are heated for 4 hours at 200° C. and 500 g of technical cresol mixture, whereby a portion of the cresol is distilled off. After cooling and filtering the obtained brown crystals are washed with alcohol and ether.

The product does not melt at below 360° C.

EXAMPLE F

334 g (2.44 mol) of p-aminobenzoic acid are dissolved in 1700 cc of warm technical cresol mixture. This solution is added to a warm solution of 446 g (2.44 mol) of trimellitic acid anhydride in 1300 c of technical cresol. These combined solutions are then heated to refluxing for 1 hour. After cooling and filtering the very finely granular white powder is washed with alcohol and ether.

The product does not melt at below 360° C.

Titration of the acid in dimethyl formamide with water free sodium methylate solution with cresol red as indicated gives an equivalent weight of 153 (calculate value 155.5).

EXAMPLE G

75 g (1 mol) of glycocol and 500 cc of technical cresol are heated to 100° C. To the obtained suspension is added in small portions 109 g (0.5 mol) of pyromellitic acid anhydride. The mixture is then further heated to 200° C., whereby water distills off. After cooling the finely crystalline residue is filtered off and washed with methylene chloride. After recrystallization from 1,4-dioxane the product which is obtained is in the form of yellowish-white crystals and does not melt up to temperatures of 320° C.

EXAMPLE H

334 g (2.44 mol) of p-aminobenzoic acid is dissolved under warming in 1300 cc of technical cresol mixture and the solution is added to a hot solution of 266 g (1.22 mol) of pyromellitic acid dianhydride in 800 cc of cresol. The mixture is heated for 5 hours at 190° C., whereby 50 cc of water distilled off. The precipitated white precipitate is filtered off after cooling and washed with methanol and water.

EXAMPLE J

75 g (1 mol) of glycocol are mixed with 200 g of technical cresol to form a slurry and 192 g (1 mol) of trimellitic acid anhydride are added to the suspension warmed to a temperature of 100° C. Upon heating of the mixture water distills off and at 200° C. a clear solution is obtained from which a yellowish precipitate falls upon cooling.

The obtained suspension is stirred with a large amount of acetone and the insoluble portion is separated by filtration.

The obtained yellowish white crystalline powder melts at 257° C.

EXAMPLE K

218 g (1 mol) of pyromellitic acid anhydride are dissolved in 400 cc of dimethyl formamide at 80° C. under nitrogen atmosphere. Monoethanolamine is slowly added dropwise to this solution at the same temperature in an amount of 122 g (2 mol). The dimethyl formamide is subsequently distilled off under vacuum at a temperature between 100° and 120° C. The dirty white residue is recrystallized with the addition of activated carbon from 1,4-dioxane, whereby finely yellowish-white crystals having a melting point of 271° C. are obtained.

EXAMPLE L

218 g (1 mol) of pyromellitic acid dianhydride and 150 g (2 mol) of 3-aminopropanol are mixed with each other as described in Example H. The obtained crude product is recrystallized from ethylene glycol monomethyl ether with the addition of activated carbon. The obtained white crystalline powder melts at 234° C.

EXAMPLE M

192 g (1 mol) of trimellitic acid anhydride are dissolved under nitrogen atmosphere at 100° C. in dimethyl formamide and there is added dropwise to the solution at 60° C. 61 g (1.0 mol) of monoethanolamine. The solvent is subsequently distilled off under vacuum at temperatures of 100°-130° C. The yellowish-white residue is recrystallized two times from dioxane with the addition of activated carbon, and the compound melts at 197° C.

Instead of producing the imide group-containing compounds as described in the previous examples, to isolate the same and then to use them as starting compounds for the production of the ester imide resins according to the present invention, it is also possible, as indicated previously, to produce the imide group-containing compounds during the synthesis of the ester imide resins and to directly react the same with the polyester reactants. Essentially the same type of ester imide resins are obtained if the imide group-containing compounds are not first separately produced and isolated, but instead the compounds (a) and (b) are permitted to react in the apparatus used for the production of the ester imide resin and then directly permitted to react with the other starting materials used in the production of the ester imide resin.

The point of time at which the compounds (a) and (b) are brought to reaction is not essential. In many cases it is preferable to first produce the imide group-containing compounds by reacting (a) and (b) and then to use the resulting compound in the further production of the ester imide resin by adding this compound to the esterification reactants. It is also possible, however, to first esterify the imide group-free starting substances with each other, then to add the reaction product of compounds (a) and (b) thereto and to react these together and subsequently to cause the resulting imide group-containing compound to react with the already formed polyester resin. On the other hand it is also possible and sometimes advantageous to mix the compounds (a) and (b) with the other starting materials used for the production of the ester imide resins together and then to react all of the components together.

The following examples are given to illustrate the production and working up of the ester imide resins of the present invention. The scope of the invention is not, however, meant to be limited to the specific details of these examples.

EXAMPLE 1

A polyester is produced in known manner from 388 g of dimethyl terephthalate, 112 g of ethylene glycol and 75 g of glycerin and this polyester is mixed in two portions with 137 g of the reaction product of Example A at a temperature of 180°-215° C. After the diimidedicarboxylic acid (reaction product as Example A) has been completely taken up in the terephthalic acid resin, 1.8 g of cadmium acetate is added thereto. The reaction mixture is subsequently further condensed for 3 hours at 215° C., finally under vacuum. The thus obtained resin is dissolved in 450 g of technical cresol while still hot and this solution is reacted with a solution of 9 g of butyl titanate in 27 g of cresol.

The resulting lacquer is then diluted with a mixture of 2 parts of solvent naphtha and 1 part of cresol to a solid body content of 37% and the resulting lacquer has a viscosity according to DIN 53 211 with a 4 mm nozzle at 20° C. of 156 seconds. (The expression "DIN" stands for Deutsche Industrienormalien which is the German Industrial Standards.).

Copper wire of 1 mm diameter is lacquered in a continuous process with this wire lacquer in accordance with the following technical data: Horizontal wire lacquering oven of 3.50 meters length, oven temperature 470° C., coating apparatus of rollers and felt, 6 coats, drawing speed of 4 m/min., coating thickness (=increase of diameter of the wire) 0.05 mm.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness       4-5 H                                               lead pencil hardness after 30 minutes                                          storage at 60° C. in benzene                                                                       3 H                                                 lead pencil hardness after 30 minutes                                          storage at 60° C. in alcohol                                                                       3 H                                                 heat shock test:                                                                           a winding of the wire                                                          around its own diameter                                                        is unobjectionable after                                                       1 hour of heating at a                                                         temperature of 155° C.                                      ______________________________________                                    

After 24 hours of ageing at 180° C. the lacquer elongation amounts to 34%. The insulation is and remains solid upon winding about its own diameter under a tension load of 0.6 kp/mm² (kp=kilopond).

EXAMPLE 2

A polyester is produced in known manner from 338 g of dimethyl terephthalate, 112 g of ethylene glycol and 75 g of glycerin, and mixed with 204 g of the reaction product of Example A at a temperature of 180°-215° C. in two portions. After the diimidodicarboxylic acid (of Example A) is completely taken up in the terephthalate resin, 1.0 g of cadmium acetate is added thereto and the reaction mixture is further condensed for 3 hours at 215° C., finally under vacuum. The obtained resin is dissolved in 450 g of technical cresol while still hot, and mixed with a solution of 9 g of butyl titanate in 27 g of cresol.

This lacquer is diluted to a solid body content of 34% and a viscosity of 120 seconds as described in Example 1, and copper wire is lacquered therewith.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in benzene 3 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 3 H                                                heat shock test:                                                                           a winding of the wire on its                                                   own diameter remains free of                                                   faults after 1 hour heating                                                    at 155° C.                                                  ______________________________________                                    

After 16 hours of ageing at 200° C. the elongation of the lacquer is 22%. The insulation remains solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 3

A polyester is produced in known manner from 388 g of dimethyl terephthalate, 112 g of ethylene glycol and 75 g of glycerin, and mixed with 273 g of the reaction product of Example A, added thereto in two portions at a temperature of 180°-215° C. Subsequently the reaction product is worked up as described in Example 1, the obtained lacquer is diluted to a solid body content of 27% and the viscosity of 55 seconds, and copper wire is lacquered therewith.

The testing of the insulation of the lacquer gives the following values:

    ______________________________________                                         lead pencil hardness       4 H                                                 lead pencil hardness after 30 minutes storage at                               60° C. in benzene   2-3 H                                               lead pencil hardness after 30 minutes storage at                               60° C. in alcohol   2-3 H                                               heat shock test: A winding of the wire about its own                           diameter of the wire prestretched 10% is free of faults                        after 1 hour tempering at 155° C.                                       ______________________________________                                    

After 24 hours of ageing at 180° C. the elongation of the lacquer amounts to 30%. The insulation remains solid upon winding about its own diameter under a tension load of 0.6 kg/mm².

EXAMPLE 4

A polyester produced in known manner from 388 g of dimethyl terephthalate, 112 g of ethylene glycol and 75 g of glycerin has 80 g of technical cresol added thereto and the mixture is heated to 80° C. 144 g of trimellitic acid anhydride and 74 g of 4,4'-diaminodiphenyl methane is then added to the resin melt and the temperature is slowly increased to 215° C. At 140° C. the resin melt becomes turbid and highly viscous, obviously due to the formation and separation of the diimidodicarboxylic acid of Example A.

Upon further temperature increase the reaction mixture slowly becomes thinly liquid again and the precipitated diimidodicarboxylic acid is gradually taken up by the resin melt. After the resin melt reaches a temperature of 215° C. it becomes clear, then 1 g of cadmium acetate is added thereto and the cresol is distilled off under vacuum at 155° C. The reaction mixture is further condensed finally for an additional 3 hours at 215° C. The obtained resin is further worked up according to Example 1 to a lacquer of 34% solid body content and the viscosity of 108 seconds, and a copper wire is then enameled therewith.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in benzene 3 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 3 H                                                heat shock test:                                                                           a winding of the wire about                                                    its own diameter is free of                                                    faults after 1 hour of temper-                                                 ing at 155° C.                                              ______________________________________                                    

After 16 hours storage at 200° C. the elongation of the lacquer amounts to 23%. The insulation is solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 5

388 g of dimethyl terephthalate, 112 g of ethylene glycol, 75 g of glycerin, 150 g of technical cresol mixture, 144 g of trimellitic acid anhydride and 74 g of 4,4'-diaminodiphenyl methane are melted together with the addition of 1 g of cerium naphthenate and stirred while heating. At about 130° C. the separation of the diimidodicarboxylic acid described in Example 1 starts. Upon further slow temperature increase methanol and water distill off. Upon reaching a temperature of 215° C. the temperature is maintained for 3 hours and the resulting resin melt becomes clear. 1 g of cadmium acetate is added thereto, the cresol is distilled off under vacuum at a temperature of 175° C., and finally condensation is continued for an additional 3 hours at 225° C.

The obtained resin is worked up as described in Example 1 to a lacquer of 34% solid body content and a viscosity of 120 seconds, and copper wire is enameled therewith.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in benzene 3 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 2 H                                                heat shock test:                                                                           A winding of the wire about                                                    its own diameter is free of                                                    faults after tempering for                                                     1 hour at 155° C.                                           ______________________________________                                    

After 16 hours of ageing at 200° C. the lacquer elongation amounts to 21%. The insulation remains solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE6

144 g of trimellitic acid anhydride, 74 g of 4,4'-diaminodiphenyl methane, and 500 g of technical cresol are heated under vigorous stirring, whereby the resulting suspension at 150° C. becomes highly viscous. Upon further temperature increase to 195° C. 27 cc of water distills off. Subsequently 200 cc of cresol distills off at 120° C. under vacuum.

To the suspension of diimidodicarboxylic acid resulting therefrom is added 388 g of dimethyl terephthalate 112 g of ethylene glycol and 75 g of glycerin along with 1 g of cerium naphthenate. The mixture is heated, whereupon methanol and water distill off. The temperature is then increased to 215° C. and maintained at this temperature until the resin melt becomes clear. After the addition of 1 g of cadmium acetate the remaining cresol is distilled off under vacuum at 170° C. and the resin melt is further condensed an additional 3 hours at 215° C.

The remaining resin is further worked up as described in Example 1 to a lacquer of 34% solid body content and a viscosity of 112 seconds, and copper wire is lacquered therewith.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in benzene 3 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 3 H                                                heat shock test:                                                                           A winding of the wire about                                                    its own diameter is free of                                                    faults after 1 hour of temper-                                                 ing at 155° C.                                              ______________________________________                                    

After 16 hours of ageing at 200° C. the lacquer elongation amounts to 24%.

The insulation remains solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 7

A polyester produced in known manner from 353 g of dimethyl terephthalate, 153 g of ethylene glycol and 88 g glycerin is heated to 80° C. with the addition of 52 g of cresol. After the further addition of 200 g of trimellitic acid anhydride and 103 g of 4,4'-diaminodiphenylmethane the reaction mixture is further heated, whereby at a temperature of 140° C. the diimidodicarboxylic acid of Example A separates. After further temperature increase esterification occurs at 215° C., until the resin becomes clear. After the addition of 350 g of technical cresol the mixture is cooled to 80° C.

Thereupon 299 g of tremillitic acid anhydride and 154 g of 4,4'-diaminodiphenylmethane are again added and the mixture is further esterified at 215° C. until a clear resin melt is obtained. The obtained product is dissolved in 1100 g of cresol and 600 g of solvent naphtha and the mixture of 20 g of butyl titanate and 60 g of cresol are added to this solution.

This solution is diluted to a solid body content of 27% and a viscosity of 123 seconds as described in Example 1 and copper wire is lacquered therewith.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in benzene:                                                                               3 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol:                                                                               3 H                                                heat shock test:                                                                           A winding of the wire about its                                                own temperature remains free of                                                faults after 1 hour of tempering                                               at 200° C.                                                  ______________________________________                                    

After 16 hours of ageing at 200° C. the lacquer elongation amounts to 20%. The insulation remains solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 8

218 g of diimidodicarboxylic acid produced according to Example A, 13 g of trimellitic acid anhydride, 31 g of ethylene glycol, 12 g of glycerin and 1 g of tin (II) oxalate in 500 g of technical cresol for 8 hours at 185° C. 225 g of cresol are then distilled off and the residue is heated for an additional 8 hours at 200° C. The obtained resin solution is diluted to a lacquer of 27% solid body content and the viscosity of 120 seconds as described in Example 1 and copper wire is lacquered therewith.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage at                               60° C. in benzene    3-4 H                                              lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 3-4 H                                              heat shock test:                                                                           A winding of the wire about                                                    its own diameter is free of                                                    faults after 1 hour of tempering                                               at 250° C.                                                  ______________________________________                                    

After 16 hours of ageing at 200° C. the lacquer elongation amounts to 27%. The insulation remains solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 9

546 g of diimidodicarboxylic acid of Example A, 43.5 g of ethylene glycol and 43.5 g of glycerin, as well as 2 g of tin (II) oxalate and 2 g of antimony (III) oxide are heated in 1000 g of o-cresol for 8 hours at 185° C., whereby the reaction water is distilled off. Subsequently 740 g of cresol are distilled off and the residue is heated for 6 hours at 200° C. The obtained resin solution is diluted as described in Example 1 to a lacquer of 29% solid body content and a viscosity of 123 seconds, and copper wire is lacquered therewith.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness     4 H                                                   lead pencil hardness after 30 minutes storage                                  at 60° C. in benzene                                                                             3-4 H                                                 lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol                                                                             3-4 H                                                 heat shock test:                                                                           A winding of the wire about its                                                own diameter is free of faults                                                 after 1 hour of tempering at                                                   250° C.                                                     ______________________________________                                    

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage at                               60° C. in benzene    3-4 H                                              lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 3-4 H                                              heat shock test:                                                                           A winding of the wire about                                                    its own diameter is free of                                                    faults after 1 hour of tempering                                               at 250° C.                                                  ______________________________________                                    

After 16 hours of ageing at 200° C. the lacquer elongation amounts to 33%. The insulation is solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 10

The procedure followed is as described in Example 2 with the exception that instead of 204 g of the diimidodicarboxylic acid of Example A, there is used 171 g of the diimidodicarboxylic acid of Example D. The obtained lacquer has a solid body content of 34% and a viscosity of 110 seconds. Copper wire is lacquered therewith.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        3 H                                                lead pencil hardness after 30 minutes storage at                               60° C. in benzene    2 H                                                lead pencil hardness after 30 minutes storage at                               60° C. in alcohol    2 H                                                heat shock test:                                                                           A winding of the wire about its                                                own diameter remains free of                                                   faults after 1 hour heating at                                                 155° C. A winding of the wire                                           about 2 times its own diameter re-                                             mains free of faults after 1 hour                                              of tempering at 200° C.                                     ______________________________________                                    

After 16 hours of ageing at 200° C. the lacquer elongation amounts to 23%. The insulation remains solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 11

A polyester is produced from 364 g of dimethyl terephthalate, 112 of ethylene glycol and 75 of glycerin and further worked up as described in Example 1, with the exception that instead of 137 g of the diimidodicarboxylic acid (of Example A) there is used 156 g of the reaction product of Example F. The obtained lacquer has a solid body content of 26% and a viscosity of 130 seconds and is used for the lacquering of wire as described in Example 1.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage at                               60° C. in benzene    3 H                                                lead pencil hardness after 30 minutes storage at                               60° C. in alcohol    3 H                                                heat shock test:                                                                           A winding of the wire pre-                                                     stretched 10% about its own                                                    diameter remains free of faults                                                after 1 hour of tempering at                                                   155° C.                                                     ______________________________________                                    

After 24 hours of ageing at 180° C. the lacquer elongation amounts to 32%. The insulation remains solid upon winding about its own diameter under a tension load of 0.6 kp/mm².

EXAMPLE 12

A polyester is produced in known manner from 388 g of dimethyl terephthalate, 112 g of ethylene glycol and 75 g of glycerin are further worked up as described in Example 1. However, instead of the 137 g of diimidodicarboxylic acid of Example A there is used 125 g of the reaction product of Example G. The obtained lacquer has a solid body content of 33% and a viscosity of 111 seconds and copper wire is lacquered therewith as described in Example 1.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness         4 H                                               lead pencil hardness after 30 minutes storage at                               60° C. in benzene     3-4 H                                             lead pencil hardness after 30 minutes storage at                               60° C. in alcohol     3-4 H                                             heat shock test:                                                                           A winding of the wire about its                                                own diameter remains free of faults                                            after 1 hour of tempering at 155° C.                        ______________________________________                                    

After 16 hours of ageing at 200° C. the insulation remains solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 13

218 g of dimethyl terephthalate, 28 g of ethylene glycol, 46 g of glycerin and 102 g of the reaction product of Example K, with the addition of 1 g of cerium naphthenate and 0.4 g of cadmium acetate, are esterified for 12 hours. During this period the temperature is gradually increased from 160° C. to 240° C. As a result the methanol bound to the dimethyl terephthalate distills off. After the end of the reaction the resin is dissolved in a mixture of 285 g of technical cresol and 185 g of solvent naphtha and stirred with 23 g of a 15% solution of butyl titanate in a cresol-solvent naphtha mixture.

The obtained lacquer of 30% solid content and a viscosity of 92 seconds is used for the lacquering of copper wire as in Example 1.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        4 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in benzene 3-4 H                                              lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 3-4 H                                              heat shock test:                                                                           A winding of the wire pre-                                                     stretched 10% about its own                                                    diameter is free of faults after                                               1 hour of tempering at 155° C.                              ______________________________________                                    

After 24 hours of ageing at 180° C. the lacquer elongation amounts to 31%. The insulation is solid upon winding about its own diameter under a tension load of 0.6 kp/mm².

EXAMPLE 14

218 g of dimethyl terephthalate, 28 g of ethylene glycol, 46 g of glycerin and 111 g of the reaction product of Example L, with the addition of 1 g of cerium naphthenate and 0.4 g of cadmium acetate are mixed with each other and reacted as described in Example 12. The obtained lacquer of 26% solid body content and a viscosity of 130 seconds is used for the lacquering of copper wire as described in Example 1.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        3 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in benzene 2 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 2 H                                                heat shock test:                                                                           A winding of the wire about                                                    its own diameter remains free                                                  of faults after 1 hour tempering                                               at 155° C.                                                  ______________________________________                                    

After 16 hours of ageing at 200° C. the lacquer elongation amounts to 22%. The insulation is solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 15

A polyester is produced from 300 g of dimethyl terephthalate, 64 g of ethylene glycol and 63 g of glycerin under the addition of 65 g of technical cresol, and 248 g of the reaction product of Example M are added at 125° C. The temperature of 190°-220° C. is used for esterification until an acid number of less than 10 is obtained. The reaction mass is subsequently further condensed an additional 90 minutes at 190°-230° C. with the distillation off of cresol under vacuum.

The resulting resin melt is mixed while still hot with 450 g of technical cresol and there is subsequently added a mixture of 11 g of butyl titanate, 17 g of cresol and 17 g of solvent naphtha and thereafter it is diluted with a mixture of equal parts of cresol and solvent naphtha to a solid body content of 25% and a viscosity of 111 seconds.

A copper wire of 1 mm diameter is lacquered with this lacquer as described in Example 1.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness         4-5 H                                             lead pencil hardness after 30 minutes storage at                               60° C. in benzene     4 H                                               lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol  4 H                                               heat shock test:                                                                           A winding of the wire about its own                                            diameter remains free of faults after                                          1 hour of tempering at 155° C.                              ______________________________________                                    

EXAMPLE 16

40 g of monoethanolamine are added dropwise to a mixture of 192 g of trimellitic acid anhydride and 500 g of technical cresol at a temperature of 20° C. under stirring, and then 450 g of cresol are again distilled off at 130° C. under vacuum. 65 g of ethylene glycol are then added and the mixture is esterified at a temperature of 160°-200° C. with a distillation off of water until the reaction product has an acid number of 3. Subsequently during a two hour time period an additional 50 g of distillate are distilled off under vacuum at a temperature of 210° C.

The residue is dissolved hot in a mixture of 450 g of cresol and 150 g of solvent naphtha and the thus obtained solution has a solid body content of 27% and a viscosity of 135 seconds.

Copper wire of 1 mm diameter is lacquered with this solution using a horizontal lacquering oven of 2.5 m length and an oven temperature of 500° C. The drawing speed amounts to 2.3 m/min. Six coatings are applied, whereupon a diameter increase of the wire of 0.045 mm is obtained.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness          4 H                                              lead pencil hardness after 30 minutes storage at                               60° C. in benzene      3 H                                              lead pencil hardness after 30 minutes storage at                               60° C. in alcohol      4 H                                              heat shock test:                                                                           A winding of the wire about its own                                            diameter is free of faults after 1 hour                                        of tempering at 155° C.                                     ______________________________________                                    

After 16 hours of ageing at 200° C. the insulation is solid upon winding about 4 times its own diameter under a tension load of 6 kp/mm².

EXAMPLE 17

116 g of trimellitic acid anhydride and 65 g of pyromellitic acid dianhydride are dissolved in 200 g of dimethyl formamide at a temperature of 100° C. and 43 g of monomethanolamine are added dropwise under stirring to the solution of 60° C. 185 g of dimethyl formamide are subsequently distilled off under vacuum at 140° C. After the addition of 56 g of ethylene glycol the reaction mixture is esterified 7 hours at a temperature from 170° C. gradually increasing to 220° C. whereby an additional 20 g of distillate are obtained.

The obtained resin is dissolved hot in a mixture of 380 g of technical cresol mixture and 130 g of solvent naphtha. The obtained lacquer has a solid body content of 38% and a viscosity of 149 seconds and is used for the lacquering of copper wire as described in Example 16.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness        5 H                                                lead pencil hardness after 30 minutes storage at                               60° C. in benzene    4 H                                                lead pencil hardness after 30 minutes storage                                  at 60° C. in alcohol 4 H                                                heat shock test:                                                                           A winding of the wire about its                                                own diameter is free of faults                                                 after 1 hour tempering at 155° C.                           ______________________________________                                    

EXAMPLE 18

192 g of trimellitic acid anhydride are dissolved in 200 g of dimethyl formamide at 90° C. and 30 g of 4,4'-diaminodiphenylmethane are added in small portions to the solution at 60° C., whereby a light yellowish precipitate gradually precipitates. 28 g of monoethanolamine are subsequently added dropwise and 170 g of dimethyl formamide are again distilled off under vacuum at a temperature of 140° C. The residue is esterified with 64 g of ethylene glycol for 6 hours, whereby the temperature during this time is gradually increased, 170° C. to 220° C. 25 g of distillate are thus obtained, and upon a subsequent vacuum treatment for 90 minutes at 220° C. an additional 25 g of distillate are obtained.

The residue is dissolved still hot in a mixture of 425 g of technical cresol and 175 g of solvent naphtha the obtained lacquer has a solid body content of 33% and a viscosity of 118 seconds and is used for the lacquering of copper wires described in Example 16.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness          4 H                                              lead pencil hardness after 30 minutes storage at                               60° C. in benzene      3 H                                              lead pencil hardness after 30 minutes storage at                               60° C. in alcohol      3 H                                              heat shock test:                                                                           A winding of the wire about                                                    its own diameter is free of faults                                             after tempering for 1 hour at 200° C.                       ______________________________________                                    

EXAMPLE 19

A mixture of 192 g of trimellitic acid anhydride and 200 g of dimethyl formamide are heated to 100° C. under stirring, whereby a clear solution results. 34 g of monoethanolamine are added thereto at 80° C. dropwise and subsequently the dimethyl formamide is distilled off at 100°-150° C. under vacuum. 52 g of neopentyl glycol and 53 g of ethylene glycol are then added thereto and the temperature is increased gradually and evenly from 180° C. during a time period of 9 hours to 220° C. with a separation of 30 g of distillate, the esterification being carried out until an acid number of 6 is obtained. The reaction mixture is then further condensed for 2 hours at 220° C. under vacuum.

The obtained resin is dissolved hot in a mixture of 500 g of technical cresol and 250 g of solvent naphtha. The lacquer has a solid body content of 28% and a viscosity of 135 seconds. The lacquer is used for the lacquering of copper wire as described in Example 16.

    ______________________________________                                         lead pencil hardness         4 H                                               lead pencil hardness after 30 minutes storage at                               60° C. in benzene     3 H                                               lead pencil hardness after 30 minutes storage at                               60° C. in alcohol     3 H                                               heat shock test:                                                                           A winding of the wire about its                                                own diameter remains free of faults                                            after 1 hour of tempering at 155° C.                        ______________________________________                                    

EXAMPLE 20

In accordance with the method described in Example 16 a resin is produced of the following starting components: 192 g of trimellitic acid, 37 g of ethylene glycol, 18 g of glycerin and 43 g of monoethanolamine. The obtained brown resin is dissolved hot in a mixture of 290 g of cresol and 90 g of solvent naphtha and the mixture is then diluted with a mixture of equal parts of cresol and solvent naphtha to a solid body content of 27% and a viscosity of 119 seconds. This lacquer is then used for the lacquering of copper wire as described in Example 16.

The testing of the lacquer insulation gives the following values:

    ______________________________________                                         lead pencil hardness         4 H                                               lead pencil hardness after 30 minutes storage at                               60° C. in benzene     3 H                                               lead pencil hardness after 30 minutes storage at                               60° C. in alcohol     3 H                                               heat shock test:                                                                           A winding of the wire about its                                                own diameter remains free of faults                                            after 1 hour of tempering at 155° C.                        ______________________________________                                    

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims. 

What is claimed as new and desired to be secured by Letters Patent is:
 1. An insulated electrical conductor comprising in combination, an electrical conductor and a cured polyester imide resin coating said electrical conductor, said polyester imide resin having ester groups of acid and alcohol moieties in the chain which are condensation products of at least difunctional carboxylic acid or a derivative thereof which is a functionally active equivalent, and at least difunctional alcohol or a derivative thereof which is a functionally active equivalent, and between said ester groups, moieties containing a 5-membered imide ring and which are condensation products of an aromatic carboxylic acid anhydride containing a 5-membered cyclic carboxylic anhydride group or a derivative thereof which is a functionally active equivalent and at least one additional reactable group, and a primary amine or a derivative thereof which is a functionally active equivalent, containing at least one additional reactable group.
 2. Insulated electrical conductor according to claim 1, said additional reactable group of the anhydride being carboxyl, carboxylic acid anhydride, or hydroxyl.
 3. Insulated electrical conductor according to claim 1, said additional reactable group of the amine being carboxyl, hydroxyl or primary amino.
 4. Insulated electrical conductor according to claim 2, said additional reactable group of the amine being carboxyl, hydroxyl or primary amino.
 5. Insulated electrical conductor according to claim 1, said alcohol moieties including residues of ethylene glycol.
 6. Insulated electrical conductor according to claim 1, said alcohol moieties including residues of glycerine.
 7. Insulated electrical conductor according to claim 5, said alcohol moieties including residues of glycerine.
 8. Insulated electrical conductor according to claim 1, the percentage of imide groups based on the sum of ester and imide groups being 9.4-50%.
 9. Insulated electrical conductor according to claim 1, the additional reactable group of the aromatic carboxylic acid anhydride being a carboxyl group.
 10. Insulated electrical conductor according to claim 1, the additional reactable group of the aromatic carboxylic acid anhydride being a anhydride group.
 11. Insulated electrical conductor according to claim 1, the additional reactable group of the aromatic carboxylic acid anhydride being a hydroxyl group.
 12. Insulated electrical conductor according to claim 1, the additional reactable group of the amine being a carboxyl group.
 13. Insulated electrical conductor according to claim 1, the additional reactable group of the amine being a hydroxyl group.
 14. Insulated electrical conductor according to claim 1, the additional reactable group of the amine being a primary amino group.
 15. Insulated electrical conductor according to claim 1, said aromatic carboxylic anhydride being trimellitic acid anhydride, pyromellitic acid anhydride or dianhydride of tetracarboxylic acid having two benzene nucleii in the molecule.
 16. Insulated electrical conductor according to claim 1, said aromatic carboxylic acid anhydride being trimellitic acid anhydride, said amine being a diprimary amine, said anhydride and amine being in the proportion of 2:1, anhydride:amine.
 17. Insulated electrical conductor according to claim 1, said aromatic carboxylic acid anhydride being trimellitic acid anhydride, said amine being a primary amino carboxylic acid, said anhydride and amine being in the proportion of 1:1.
 18. Insulated electrical conductor according to claim 1, said aromatic carboxylic acid anhydride being a tetracarboxylic acid dianhydride, said amine being a primary amino carboxylic acid, said anhydride and amine being used in the proportion of 1:2, anhydride:amine.
 19. Conductor of claim 1, the alcohol moieties including residues of polyhydric alcohols having three hydroxy groups.
 20. Conductor of claim 19, said alcohol moieties including residues of ethylene glycol.
 21. Method of coating an electrical conductor with electrically insulating enamel which comprises passing the conductor through an enameling lacquer comprising a resin for forming the enamel and a cresol type solvent, to coat the conductor with a film of the enameling lacquer, and thereafter heating the coated conductor to cure the resin and drive off the solvent, said resin being a polyester imide resin having ester groups of acid and alcohol moieties in the chain which are condensation products of at least difunctional carboxylic acid or a derivative thereof which is a functionally active equivalent, and at least difunctional alcohol or a derivative thereof which is a functionally active equivalent, and, between said ester groups, an amount effective to improve the thermal stability of the resin without rendering the resin insoluble to the extent which would make it unsuitable for use in a wire lacquer composed of the resin dissolved in cresol and solvent naphtha, of moieties containing a 5 membered imide ring and which are condensation products of an aromatic carboxylic acid anhydride containing a 5-membered cyclic carboxylic acid anhydride group or a derivative thereof which is a functionally active equivalent and at least one additional reactable group, and a primary amine or a derivative thereof which is a functionally active equivalent, containing at least one additional reactable group.
 22. Method according to claim 21, said additional reactable group of the anhydride being carboxyl, carboxylic acid anhydride, or hydroxyl.
 23. Method according to claim 21, said additional reactable group of the amine being carboxyl, hydroxyl or primary amino.
 24. Method according to claim 22, said additional reactable group of the amine being carboxyl, hydroxyl or primary amino.
 25. Method according to claim 21, said alcohol moieties including residues of ethylene glycol.
 26. Method according to claim 21, said alcohol moieties including residues of glycerine.
 27. Method according to claim 25, said alcohol moieties including residues of glycerine.
 28. Method according to claim 21, the percentage of imide groups based on the sum of ester and imide groups being 9.4-50%.
 29. Method according to claim 21, the additional reactable group of the aromatic carboxylic acid anhydride being a carboxyl group.
 30. Method according to claim 21, the additional reactable group of the aromatic carboxylic acid anydride being an anhydride group.
 31. Method according to claim 21, the additional reactable group of the aromatic carboxylic acid anhydride being a hydroxyl group.
 32. Method according to claim 21, the additional reactable group of the amine being a carboxyl group.
 33. Method according to claim 21, the additional reactable group of the amine being a hydroxyl group.
 34. Method according to claim 21, the additional reactable group of the amine being a primary amino group.
 35. Method according to claim 21, said aromatic carboxylic anhydride being trimellitic acid anhydride, pyromellitic acid anhydride or dianhydride of tetracarboxylic acid having two benzene nucleii in the molecule.
 36. Method according to claim 21, said aromatic carboxylic acid anhydride being trimellitic acid anhydride, said amine being a diprimary aromatic amine, said anhydride and amine being in the proportion of 2:1, anhydride:amine.
 37. Method according to claim 21, said aromatic carboxylic acid anhydride being trimellitic acid anhydride, said amine being a primary amino carboxylic acid, said anhydride and amine being in the proportion of 1:1.
 38. Method according to claim 21, said aromatic carboxylic acid anhydride being a tetracarboxylic acid dianhydride, said amine being a primary amine carboxylic acid, said anhydride and amine being used in the proportion of 1:2, anhydride:amine.
 39. Method of claim 21, the alcohol moieties including residues of polyhydric alcohols having more than two hydroxy groups.
 40. Method of claim 39, said alcohol moieties including residues of ethylene glycol.
 41. Method of claim 21, the alcohol moieties including residues of polyhydric alcohols having three hydroxy groups.
 42. Method of claim 21, said alcohol moieties including residues of ethylene glycol and residues of alcohols having three hydroxy groups. 